This invention relates to a waterproofing device for an outboard engine control circuit. The control circuit, which is often installed outboard, together with the drive mechanism for lifting and lowering the propeller, is subject to troubles due to the frequent exposure to seawater splashes. Particularly, a waterproofing construction is essential in a circuit breaker designed to forcibly open the circuit to protect from burnout when an excess current flows due to a trouble or a wrong wiring.